Transport refrigeration systems for conditioning the loads of trucks and trailers have cooling, null and heating modes. The heating mode includes a heating cycle for controlling load temperature to a selected set point, as well as a heating cycle for defrosting the evaporator coil. When the system switches from a cooling or null mode into a heating cycle, hot compressor discharge gas is diverted by suitable mode selecting valve means from the normal refrigerant circuit, which includes a condenser, receiver, expansion valve, evaporator, and accumulator, to a heating cycle circuit which includes a the compressor, the evaporator, and the accumulator.
To make more liquid refrigerant available during a heating cycle, a prior art procedure pressurizes the receiver with hot compressor discharge gas to force liquid refrigerant out of the receiver and into the refrigerant cooling circuit. This requires an auxiliary hot gas line which runs from the main hot gas line to the receiver, along with a by-pass check valve, a by-pass service valve, a receiver tank pressure solenoid, and a condenser check valve. A bleed port in the expansion valve allows the liquid refrigerant forced out of the receiver to flow into the evaporator during the heating cycle, to improve heating and defrosting capacity.
U.S. Pat. No. 4,748,818, which is assigned to the same assignee as the present application, improved upon the aforesaid prior art procedure by connecting the output of the receiver to the accumulator during a heating cycle. This eliminated the auxiliary hot gas pressure line to the receiver, and the hereinbefore mentioned associated control items. While this arrangement allowed some refrigerant to flow from the condenser to the receiver, some refrigerant was still being trapped in the condenser, especially at low ambient temperatures.
U.S. Pat. No. 4,912,933, which is assigned to the same assignee as the present application, improved upon the arrangement of the '818 patent by connecting the receiver to the accumulator when the need for a heating cycle is detected, with the connection being made before the mode selecting valve means actually switches to the heating circuit. In other words, the need for a heating cycle establishes direct refrigerant flow communication between the receiver and accumulator while delaying the switch of the hot gas refrigerant flow from the cooling circuit, which includes the condenser and receiver, to the heating circuit. This forces a substantial amount of the refrigerant trapped in the condenser and receiver to flow to the lower pressure accumulator, providing a significant enhancement to the heating and defrost cycles. In both the '818 and '933 patents, the direct fluid flow communication between the receiver and accumulator is preferably maintained during the heating cycle, with a check valve preventing reverse flow into the receiver.
U.S. Pat. Nos. 4,685,306; 4,706,468; 4,711,095; 4,712,383; 4,896,512; and 4,932,219, which are assigned to the same assignee as the present application, are all directed to compartmentalized or multi-temperature transport refrigeration systems which have at least first and second served spaces or compartments which are individually conditioned to selected set point temperatures. These patents mention the problem of obtaining sufficient heating capacity at low ambient temperatures in a compartmentalized transport refrigeration system. It is common when such systems are to be utilized in extremely cold ambient temperatures to employ an induction generator to power electrical heating resistors, instead of, or to augment, hot refrigerant gas heating. The system of the '933 patent has not been utilized in a compartmentalized transport refrigeration unit because while one compartment is in a heating cycle, another may be in a cooling cycle, and it was not thought that the condenser flushing of the '933 patent could be economically employed.
It would be desirable, and it is an object of the present invention, to be able to sufficiently enhance the hot refrigerant gas heating capability of compartmentalized transport refrigeration systems to the point where the need for auxiliary heating, such as provided by ah induction generator, may be eliminated, or at least significantly reduced.